The following is a ANSI Standard Pipe Thread Recommended Tap size and Drilled Pilot Hole for Taper

National Pipe Thread Taper ( NPT ) is a U.S. standard for tapered threads used on threaded pipes and fittings . In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tight seal.

Threaded pipes can provide an effective seal for pipes transporting liquids , gases , steam , and hydraulic fluid. These threads are now used in materials other than steel and brass, including PTFE , nylon , bronze , and cast iron .

The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr-free and lubricated using a lubricating material like lubricating paste or tape . The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.

Commonly used sizes are 1/⁄ 8 , 1/4 , 3/⁄ 8 , 1/2 , 3/4 , 1, 1 1/⁄ 4 , 1 1/2 , and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Sizes smaller than 1 ⁄ 8 inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon, due to the use of alternative methods of joining that are used with these larger sizes.